This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7119 arising from an application entitled, CHANNEL COMPENSATOR FOR DS-CDMA RECEIVER, earlier filed in the Korean Industrial Property Office on Sep. 3, 1998, and there duly assigned Serial No. 1998-36248.
1. Field of the Invention
The present invention relates to mobile communication systems which employ direct sequence-code division multiple access (DS-CDMA) for facilitating coherent communication reception, and in particular to a channel compensator for estimating and compensating for phase changes and residual frequency offsets of despreaded signals prior to synchronous demodulation.
2. Description of the Related Art
In a wireless communication system, a communication signal is received through several independent paths. A receiver element receives multiple versions of the transmitted signals that have propagated along different paths. However, the strength of received signals may vary due to complex addition of multipath signals known as fading. Among the technique used to mitigate the effects of fading in DS-CDMA communication system is the space diversity technique. Space diversity reception methods can be classified into four categories; 1)selection diversity; 2)feedback diversity; 3)maximal ratio combining, and 4)equal gain combiner. Depending on the type of methods utilized, the performance of the system varies.
The diversity technique used in an IS-95 DS-CDMA system often uses a RAKE receiver employing a Maximal Ratio Combining (MRC), which is considered to be most superior in the performance by enabling received signals to better withstand the effects of various channel impairments. The RAKE receiver, which is well known in art, employs synchronous demodulation to provide an improvement in the communication link between a transmitter and a receiver by recreating more accurately the informational content of the communication signal actually formed at the transmitter, through time diversity technique. Two-phase or binary Phase Shift Keying (PSK) is the most suitable modulation scheme because of its high power efficiency at high bit error rates (BERs). In order to make high quality synchronous demodulation of PSK possible, it is desirable to compensate for phase information such as a Rayleigh fading""s phase change, and for frequency offsets from the transmitter/receiver clock not being perfectly locked due to inaccuracies in the local oscillator. Thus, any means to improve the quality of the channel estimate would therefore be beneficial to facilitate better recreation at the receiver of a signal transmitted thereto.
A synchronous DS-CDMA system uses an average value of a received signal, to compensate for phase shifts and different amplitude attenuations due to fading in a radio mobile channel. However, the average value of a particular sample duration is not adopted to compensate for any variation in channel environment or channel characteristics which may affect the information signal communicated from a transmitter to a receiver.
A Maximum Likelihood (ML) algorithm can be used to observe frequency offsets during the time-varying phase frequency period. The ML algorithm using the average value of the received signal typically includes a joint estimation of an attenuation vector xcex1 and a phase vector xcex8. From the maximized conditional probability density function p(xcfx81|xcex1, xcex8, Tp) for a particular observation xcfx81 of the received signal vector rxe2x80x94k=(rxe2x80x94k,l, . . . , rk, L)T, the following is obtained:                                                         α              l                        _                    ⁢                      xe2x80x83                    ⁢          cos          ⁢                      xe2x80x83                    ⁢                      (                                          θ                l                            _                        )                          =                              1                          N              p                                ⁢                                    ∑                              n                =                0                                                              N                  p                                -                1                                      ⁢                          xe2x80x83                        ⁢                          Re              ⁢                              {                                                                            r                      _                                                              k                      ,                      l                                                        ⁡                                      [                    n                    ]                                                  }                                                                        (        1        )                                                                    α              l                        _                    ⁢                      xe2x80x83                    ⁢          sin          ⁢                      xe2x80x83                    ⁢                      (                                          θ                l                            _                        )                          =                              1                          N              p                                ⁢                                    ∑                              n                =                0                                                              N                  p                                -                1                                      ⁢                          xe2x80x83                        ⁢                          Im              ⁢                              {                                                                            r                      _                                                              k                      ,                      l                                                        ⁡                                      [                    n                    ]                                                  }                                                                        (        2        )            
where Np is an observation length in channel estimation.
By using equations (1) and (2), the receiver can obtain the residual frequency offsets generated by the channel at a particular sample duration indicative of the time-varying phase and frequency.
The following discussion of various figures give some background as to system and methods involved to a rake receiver scheme for receiving signals through a multipath environment.
FIG. 1 illustrates a structure of a common known receiver in a mobile station. It will be appreciated by those skilled in the art that several different receivers exists which can be used to retrieve transmitted signal from the communication channel. In FIG. 1, an input signal, which is down converted to a baseband signal by a local oscillator, is sampled at a particular chip rate, and a searcher is used to look for alternate multipaths and for neighboring base station signals to find a strong correlation with the assigned code. Once a strong signal is located at a particular time offset, the searcher assigns a receiver element to demodulate that signal. The input signals from L multipath are combined with a diversity combiner for preventing a performance degradation due to the multipath fading.
FIG. 2 illustrates a structure of the finger of a conventional mobile station for a particular path l mentioned in the above. A pair of PN sequences are generated by I-channel PN generator and Q-channel PN generator. In FIG. 2, the sampled signals are despread by local PN (Pseudo Noise) codes aI and aQ so that the phase change and the residual frequency offset of the despread signals are compensated by channel compensators 100. Thereafter, by multiplying the channel compensated signals by a Walsh code b(n), wherein the spreading codes are orthogonal to each other, the received signal can be correlated with a particular user spreading code such that it would possible to acquire only a desired logical channel or user signal related to the particular spread code, such as a sync channel, a paging channel, and a traffic channel by correlating the received signals. At the next stage, an integrator for integration of the multipath correlation component over a predefined interval of time converts the signals in the chip unit to the signals in the bit unit in order to obtain a processing gain by the PN code. The ratio of the spread data rate to the initial data rate is called the processing gain. The correlator use the processing gain to recover the desired signal and reject the interference. It will be appreciated by those skilled in the art that the integrator function may be implemented with a data sample summing circuit and multiplier.
FIG. 3 illustrates a structure of a conventional channel compensator 100 incorporating the ML algorithm for performing the channel compensation based on an integration value for a predefined interval of time. FIG. 4 illustrates the compensation procedure by the conventional channel compensators, in which N denotes the number of chips per bit, and Np denotes observation duration which corresponds to elapsed time for updating the estimation value (*phase and amplitude) to a total observation duration.
FIG. 5 illustrates a graph showing a total Eb/lo for 2% error, frequency offsets, and Doppler frequency for a single integrator structure, when Np=512 and 3 paths are provided. Here, Eb denotes for an energy of one bit, and lo denotes for all the signals that the mobile station has received through its antenna. As illustrated in the graph, when the frequency offset and the Doppler frequency are both high, the Eb/lo value is also very high for providing a required performance by the receiver. However, when the signal power is low, the Eb/lo value becomes smaller.
As illustrated in the above, a rake receiver uses a channel estimator for preventing the phase change and frequency offsets. The channel estimator includes a single integrator structure and uses the ML algorithm to obtain the time-varying phase information by measuring an average value for a particular duration of sampling data. Although the channel estimator with a single integrator structure has both the noise suppression capability and the channel estimation capability, these two capabilities are independent to each other, and thereby it is not the optimal means to provide a coherent communication reception. That is, if Np is to be great, the noise suppression capability is improved, but the channel estimation capability is deteriorated. In contrast, if the Np is to be small, the channel estimation is improved, and the noise suppression capability is deteriorated.
Therefore, it is necessary to provide a different scheme of observation duration to achieve higher optimal system performance. That is, the conventional channel estimator with the single integrator is susceptible to the channel environment change, resulting in a serious performance degradation due to an abrupt channel change cause by variations in multipath characteristics. Arguably, an optimal observation duration can be set for a particular channel environment using a single integrator; however, it is still impossible to avoid the performance degradation since the channel environment is variously changing in time and space.
It is therefore an object of the present invention to provide a channel compensator for estimating and compensating for a phase change and a residual frequency offset prior to decoding so that a DS-CDMA receiver may be less susceptible to variation of multipath characteristics due to unpredictable changes in the channel environment.
To achieve the above object, there is provided a channel compensator for compensating for a phase change and a residual frequency offset of a despreaded signal prior to synchronous demodulation in a DS-CDMA (Direct Sequence-Code Division Multiple Access) receiver. The channel compensator includes a first integrator for accumulating input signals sampled at a given chip rate for a predetermined period, Np1; a shift register including a plurality of registers for shifting the data output from the first integrator; a second integrator for integrating the data generated from the respective registers of the shift register; a delay means for delaying the input signal so that the input signal reaches a central register of the shift register; and, a multiplier for multiplying the delayed input signal with the output from the second integrator.
The delay means delays the input signal by a time required for the input signal accumulated in the first integrator to reach the central register portion of the shift register through the first integrator. The second integrator includes a summer for adding the values of the respective registers of the shift register at update period in relation to the first integrator, and a multiplier for multiplying the output value from the summer by a reciprocal number indicative of the number of the registers in order to reduce the dispersion of an estimation value to noises, and also to adjust the bit value for the detected information from the shift register.